Is visceral fat a better predictor of the incidence of impaired glucose tolerance or type 2 diabetes mellitus than subcutaneous abdominal fat: a systematic review and meta-analysis of cohort studies.

BACKGROUND: Several lines of evidence show that abdominal fat is strongly associated with insulin resistance and dysglycemia (impaired glucose tolerance - IGT or type 2 diabetes mellitus - T2DM). However, which component of abdominal fat, subcutaneous or intra-abdominal, has a major impact on the development of insulin resistance and dysglycemia is still a matter of debate. The aim of this review is to summarize the best available evidence on the contribution of subcutaneous and/or intra-abdominal adipose tissues to the incidence of impaired glucose tolerance and/or type 2 diabetes mellitus, in adults as well as to determine which type of abdominal fat is a better predictor of these metabolic disorders. METHODS: A search of published articles on PUBMED (1966 to June 2013), EMBASE (1980 to June 2013), LILACS (1982 to June 2013) and Central Cochrane databases was conducted to identify studies evaluating the relationship between intra-abdominal and/or subcutaneous adipose tissue and the incident IGT or T2DM). Cohort studies examining the association between intra-abdominal and/or subcutaneous adipose tissue values and the prospective development of impaired glucose tolerance or type 2 diabetes mellitus (estimated risk) were included in this review. Data extraction and risk of bias assessments were performed in duplicate by 2 reviewers. Random-effects meta-analyses were performed to pool OR estimates from individual studies to assess the association between intra-abdominal and/or subcutaneous adipose tissue values at baseline and the risk of development of impaired glucose tolerance or type 2 diabetes mellitus. Statistical heterogeneity was assessed using the I2 statistics. The risk of bias was assessed by examining the sample selected, recruitment method, completeness of follow up and blinding according to the guidelines for assessing quality in prognostic studies proposed by Hayden (29) and the MOOSE (30) statement, and adapted by us. RESULTS: Five relevant studies were suitable for this review. The analysis showed that both VAT and abdominal SAT measurements at baseline were strong predictors of incident impaired glucose tolerance or type 2 diabetes mellitus, in minimally adjusted models. However, when other confounding variables besides age, sex and ethnicity were taken into account, VAT, but not SAT, measurements pose a high risk of the incident IGT or T2DM in a wide range of age and ethnic backgrounds (Japanese-, Hispanic-, African-Americans and Canadians). CONCLUSIONS: In conclusion, the present results provide some evidence that VAT imposes more risk to the development of IGT and T2DM than abdominal SAT. However, more studies are necessary to confirm these results and to address the issue of changes in VAT and abdominal SAT and their predictive value regarding IGT and type 2 diabetes developments.